1. Field of the Invention
This invention relates to compressors which use reed valves, e g., spring or flapper valves, to control the discharge of a compressed fluid, and more particularly to compressors in which it is necessary to reduce noise and vibrations.
2. Description of the Related Art
It is known in the related art that compressors which compress fluid have a tendency to generate noise and vibration. A major cause of this noise generation is fluid over pressures generated during the compression process. Compression of the fluid takes place when the fluid is admitted into the compression chamber, e.g., the cylinder or pocket(s), and reduced in volume by the compressor. After compression, the fluid is discharged into the discharge chamber. In order to prevent the fluid from returning to the compression chamber, the compressor may be equipped with a reed valve which is positioned to close the discharge port. This valve opens or closes in response to the degree of compression of the fluid, i.e., the difference in pressure between the compression chamber and the discharge chamber across the valve. Such valves are described in U.S. Pat. No. 4,095,921 to Hiraga et al., the disclosure of which is incorporated herein by reference.
Such compressors also often are supplied with a lubricating oil which is fed into the compression chamber with the fluid to be compressed. This oil lubricates the operating components of the compressor and is essential for the longevity of the compressor. This oil, however, coats all working components of the compressor, including the reed valve(s) and the discharge port(s). Moreover, the temperature of the introduced fluid remains lower due to reduced friction, and the compressor parts are kept cooler and, therefore, last longer.
As described in U.S. Pat. No. 4,095,921, multi-cylinder compressors for the compression of fluids, such as refrigerants, are well known. The fluid introduced to the cylinders is compressed by means of a plurality of pistons which respectively reciprocate within the cylinders. In one type of compressor, the reciprocation of the pistons is produced by the cooperation of a rotating cam rotor and a wobble plate, which converts the rotation of a drive shaft to the desired reciprocation of the pistons. Slant plate-type compressors, such as swash or wobble plate-type compressors, which reciprocate pistons by converting the rotary motion of a cam rotor into nutational motion of a wobble plate are well known in the art. Such variable displacement wobble plate compressors are disclosed in Japanese Patent Application Publication No. 58-158382. Changing the angle of inclination of the wobble plate changes the stroke of the pistons and, therefore, changes the displacement volume of the cylinders.
In slant plate type compressors, both wobble plates and swash plates are disposed at a slant angle and drivingly couple the pistons to the drive shaft. Nevertheless, wobble plates nutate only. Swash plates both nutate and rotate. The term slant plate-type compressor will be used to refer to any type of compressor, including wobble and swash plate-types, which uses a slanted plate or surface in the drive mechanism. In reciprocating compressors, the pistons are coupled with a drive mechanism to convert the rotary motion of a drive shaft to reciprocating motion of each of the pistons and moves the pistons between axially spaced first or up-stroke and second or down-stroke positions, i.e., the "dead center positions." As a result, during one complete rotation of the slant plate, the piston moves in one direction, toward the first position, during which the volume of the cylinder(s) is increased, and the pressure in the cylinder(s) drops. During this movement, fluid enters the cylinder(s) via the suction port(s). When the piston moves in the opposite direction, toward the second position, during which the volume of the cylinder(s) is reduced, and the pressure in the cylinder rises; fluid is discharged from the cylinder(s) via the discharge port(s). In this type of compressor, the pistons come as near as possible to the valve plate, i.e., obtain as small as possible a tolerance between the piston and the valve plate, when the piston is at the second position. Nevertheless, the piston should not come into contact with valve plate. A minimum permissible tolerance must be maintained between the piston and the cylinder and the piston and the valve plate when the compressor is assembled, in order to achieve maximum compression efficiency.
As disclosed in U.S. Pat. No. 4,893,993 which is incorporated herein by reference, a conventional slant plate-type compressor includes a compressor housing, a front end plate, and a cylinder head. A cylinder block is formed in the compressor housing. The front end plate is attached to one end surface of the compressor housing to cover the opening of the housing. The cylinder head is disposed on the other end surface of the compressor housing and is attached to the cylinder block through a valve plate. A suction chamber and a discharge chamber are formed within the cylinder head adjacent to the valve plate. A plurality of cylinders are formed in the cylinder block, and pistons are reciprocatingly placed with the cylinders. A drive shaft extends within the compressor housing and is rotatably supported within an opening in the front end plate through a bearing. A drive mechanism for reciprocating the pistons is mounted on the drive shaft in the crank chamber. The drive mechanism includes a rotating portion and a mechanism for converting rotary motion into reciprocating motion. Further, each piston may be coupled to the drive mechanism through a connecting rod.
If the pressure in the cylinder greatly exceeds the desired discharged fluid pressure, peripheral equipment, e.g., receiver/dryer units and condensers, may be subjected to excessive pressures and damaged. Over pressures of about 30% may occur at a discharge pressure of about 350 psig, and about 60% at about 150 psig. Further, over pressures may generate noise because the reed valve(s) is brought into violent contact with the reed stops. See U.S. Pat. No. 4,095,921. The over pressure may result in noise generated by the throttling effect of the reed valve(s) on the discharge port(s) or reed valve flutter or "slapping" as the reed valve(s) strikes the valve stops. Such over pressures are due in large part to the presence of lubricant, e.g., lubricant mist, in the fluid to be compressed. When a reed valve is in the position in which it closes the discharge port, lubricant may be held between the reed valve and the valve plate and may generate an adhesive force between the valve and the plate. As a result, the reed valve may adhere to the valve plate and prevent the immediate release of pressure from the compression chamber. Although some adhesive force may be needed to maintain a desired seal between the reed valve(s) and the valve plate and to obtain a desired sealing of the cylinders, this force may become too large because the valve plate has a very smooth surface, e.g., in a range of about 0.2 to 0.8 .mu.m R.sub.z (R.sub.z is the average height of surface peaks).
In order for the reed valve(s) to open and allow fluid to flow from the cylinder into the discharge chamber, the force applied to the valve(s) by the fluid must over come the adhesive and mechanical forces affecting the reed valve(s). The mechanical force must be applied to the reed valve(s) is determined in part by the physical dimensions of the valve(s) and the mechanical properties of the valve material, e.g., Young's Modulus (resiliency). The adhesive force between the reed(s) valve and the valve plate may be determined in part by the thickness of the oil film between the contacting surfaces of the valve(s) and the plate, the viscosity of the lubricating oil, and the area of the valve(s) in contact with the plate.
The combination of these forces must be overcome in order to open the valve(s). As mentioned above, it is the pressure of the fluid acting on the valve(s) that causes it to open. Over pressurization of the fluid occurs when the combination of these forces becomes large. See FIG. 7. This over pressure can not only cause high noise levels within the compressor, but also contributes to excessive wear on and wasted energy by the compressor. Excessive wear contributes to reduced compressor life, and the wasted energy generates heat which causes a lower Isentropic efficiency. Nevertheless, all the above parameters contributing to this situation are fixed for a specific type of compressor other than the surface finish of the valve(s). Further, although the discussion above has focused on reciprocating compressors, the same principles hold true for rotary compressors, such as vane and rolling piston compressors, and orbiting compressors, such as scroll compressors, that use discharge reed valves. Such a rotary compressor is described in U.S. Pat. No. 4,900,238 to Shimizu et al., the disclosure of which is incorporated herein by reference.